Wii if you like Metroid, Smash Bros, Okami, Mario Kart/Mario Galaxy/Mario Party, Trauma Center, No More Heroes, and probably a few more things. Also good if you have friends over a lot. Zelda Wii is coming out too. The good games on Wii are exceptional.
360 for Guitar Hero/Rock Band (good party game and better DLC support), Infinite Undiscovery(?), Oblivion, Mass Effect (M.E.2 coming soon), Ninja Gaiden 2, Final Fantasy XIII soon, and a bit more. Also a lot of shooters if you're into that sort of thing. Generally a more solid library in my opinion, but Guitar Hero/Rock Band might be your only party game. They're great, but are also expensive and take quite a bit of space.